1. Field of the Invention
The present invention relates to a classifier and a method for preparing a toner. More specifically, the present invention relates to a classifier for classifying particles to obtain toner particles with a desired particle diameter in the process of preparing a dry toner, which toner is used to develop latent electrostatic images into visible toner images, particularly in the fields of electrophotography, electrostatic recording, and electrostatic printing.
2. Discussion of Background
A conventional classifier for separating a solid powder material with a particle size in the order of micron into fine particles and coarse particles is composed of a cylindrical dispersion chamber and a classification chamber. A conical member is disposed between the dispersion chamber and the classification chamber. The solid material is fed into the dispersion chamber through a feed inlet formed at an outer upper end portion of the dispersion chamber. The solid material undergoes a dispersion operation in a stream of cyclonic air introduced into the dispersion chamber, and is then introduced into the classification chamber where the solid material is subjected to centrifugal classification, so that the solid material is separated into fine particles and coarse particles, which are then respectively discharged from a fine particle discharge outlet and from a coarse particle discharge outlet.
FIG. 6 is a schematic cross sectional view of a conventional classifier, showing the structure thereof.
The classifier shown in FIG. 6 is composed of a feed pipe 1 for feeding a solid material and a stream of transport air serving as primary transport air stream for transporting the solid material into a dispersion chamber 3; an exhaust pipe 2 for discharging ultrafine particles together with air; the dispersion chamber 3; an air flow-in inlet 4 through which air serving as secondary transport air is to be fed into the dispersion chamber 3 is caused to flow in; a fine particle discharge outlet 5 from which fine particles are discharged together with air; a coarse particle discharge outlet 6 from which coarse particles are discharged together with air; a conical member/disposed at a lower portion of the dispersion chamber 3 for increasing the cyclonic flow of the solid material within the dispersion chamber 3; a classification plate 8 disposed under the conical member 7; and a classification chamber 9 formed so as to be enclosed with the conical member 7 and the classification plate 8. The above-mentioned conventional classifier is provided in its entirely in a substantially cylindrical housing.
The operation of the conventional classifier shown in FIG. 6 will now be explained.
To begin with, air is introduced into the dispersion chamber 3 and the classification chamber 9 from the feed pipe 1 and from the air flow-in inlet 4, and at the same time, the introduced air is discharged from the dispersion chamber 3 and from the classification chamber 9 through the fine particle discharge outlet 5 and the coarse particle discharge outlet 6, whereby a cyclonic air stream is formed within both the dispersion chamber 3 and the classification chamber 9.
With the formation of the cyclonic air stream within the dispersion chamber 3 and the classification chamber 9, a solid material is introduced into the dispersion chamber 3 together with air through the feed pipe 1. In the dispersion chamber 3, the solid material is rotated and caused to fall down while being subjected to centrifugal force by the cyclonic air stream. In the course of the falling down of the centrifuged solid material, ultra-fine particles of the solid material with an extremely small particle size are led toward a central portion of the dispersion chamber 3 and discharged outside through the exhaust pipe 2 which is connected to a suction device such as a suction fan (not shown).
The solid material, while rotating and falling in the dispersion chamber 3, is led into the classification chamber 9 through a ring-shaped slit A. In the classification chamber 9, the solid material again undergoes centrifugation. In the course of the centrifugation, coarse particles of the solid material are moved away from the central portion of the classification chamber 9 by centrifugal force, and are caused to pass through a ring-shaped slit B which is formed between the classification plate 8 and the inner wall of the classification chamber 9, and are finally discharged outside from the coarse particle discharge outlet 6, for example, with the aid of a suction fan (not shown).
On the other hand, fine particles of the solid material are attracted to the central portion of the classification chamber 9 by centripetal force, and are then discharged outside through the fine particle discharge outlet 5 which is connected to a suction device such as a suction fan (not shown).
For use in such a conventional classifier as mentioned above, there is proposed a method of preventing an aggregate from mixing with the solid material which is led into the classification chamber, for instance, in Japanese Laid-Open Patent Application 10-43692. In the Japanese Laid-open Patent Application, there is disclosed a classifier comprising a rotor for producing the cyclonic air stream, which rotor is disposed at an upper portion of the dispersion chamber, thereby preventing the particles of the solid material from aggregating in the dispersion chamber and improving the yield of the product.
The above-mentioned conventional classifier is capable of preventing the aggregation of the particles of the solid material by the provision of the rotor for producing the cyclonic air stream. However, it is not always easy to provide such a rotor.
Furthermore, the conventional classifier has two major problems to be tackled.
One problem is that there must be improved the dispersing performance for the solid material introduced into the dispersion chamber. It will be ideal that the particles of the solid material individually smoothly pass through the dispersion chamber and are then subjected to centrifugal classification in the classification chamber. However, there is a case where the particles interact to form aggregates while the particles descend in the dispersion chamber, and continually stay or reside, whirling, even in an upper portion of the classification chamber. This will bring about a significant reduction in the classification accuracy.
The other problem is that there must be improved the classification accuracy of the classification chamber.
Ideally, the solid particles led into the classification chamber from the dispersion chamber would be classified, for example, in such a manner that the solid particles with a desired particle diameter or more are all collected as coarse particles and the solid particles with a particle diameter less than the desired particle diameter are all collected as fine particles. However, in the conventional classifier, there occurs a problem that part of the particles having the particle diameters larger than the desired particle diameter are collected as the fine particles, while part of the particles having particle diameters smaller than the desired particle diameter are collected as the coarse particles. Therefore, a classifier capable of classifying the particles with a minimum classification inaccuracy and a sharp particle size distribution is in demand.
It is therefore a first object of the present invention to provide a classifier which is capable of solving the above-mentioned problems in the conventional classifier, improving the particle dispersion performance of the dispersion chamber by structural modification of the classifier, which can be carried out without difficulty, and also improving the classification accuracy in the classification chamber, thereby separating particles with particle diameters within a desired range, with high efficiency.
A second object of the present invention is to provide a method of producing a toner having a desired particle diameter using the above-mentioned classifier.
The first object of the present invention can be achieved by a classifier comprising:
a dispersion chamber for dispersing a powder material therein which is fed thereinto together with a stream of transportation gas through a feel inlet so as to cause a cyclonic flow of the powder material within the dispersion chamber, with finely-divided particles with particle diameters less than a predetermined particle diameter contained in the powder material being separated and discharged therefrom by means of centripetal force,
a classification chamber connected to the dispersion chamber so that the powder material fee of the finely-divided particles is fed thereinto from the dispersion chamber, which classification chamber is capable of classifying the power material free of the finely-divided particles into fine particles and coarse particles by means of centrifugal force, and
a conical member disposed between the dispersion chamber and the classification chamber which is capable of serving as a partition therebetween and enhancing the cyclonic flow of the powder material within the dispersion chamber,
wherein the dispersion chamber comprises particle residence prevention means for preventing the powder material from residing within the dispersion chamber by changing the speed of the cyclonic flow of the powder material in the dispersion chamber so as to be decreased in the direction of the feed inlet within the dispersion chamber.
In the above-mentioned classifier, the particle residence prevention means may comprise a cylindrical chamber which constitutes an upper part of the dispersion chamber, with the upper base portion of the cylindrical chamber being made smaller in size than the lower base portion thereof, and the feed inlet being disposed at the smaller upper base portion of the chamber.
In the above-mentioned classifier, the cylindrical chamber of the particle residence prevention means may also be in the shape of a circular truncated cone having such a side wall that is inclined at an angle of xcex1 with respect to a horizontal direction of the base portion of the chamber, where 0xc2x0 less than xcex1 less than 90xc2x0, or the cylindrical chamber of the particle residence prevention means may have a curved side wall, whereby the dispersion performance for the powder material attained by the dispersion chamber, and the classification accuracy for the powder material attained by the classification chamber can be improved.
In the above-mentioned classifier, it is preferable that the angle xcex1 be in a range of 30xc2x0xe2x89xa6xcex1 less than 90xc2x0, since the particle residence prevention effect of the particle residence prevention means can be improved by setting the angle xcex1 in the range.
In the above-mentioned classifier, it is preferable that the particle residence prevention means be constructed so as to be detachable from the dispersion chamber. This is because the conditions for the classification, such as the above-mentioned angle xcex1, can be changed, and the time required for changing the conditions for the classification can be shortened.
The above-mentioned classifier may comprise a plurality of feed inlets for feeding the powder material into the dispersion chamber by providing at least one additional feed inlet in addition to the feed inlet, whereby the powder material can be subdivided and fed so as to reduce the interaction of the particles of the powder material and accordingly the dispersion performance of the dispersion chamber and the classification accuracy of the classification chamber can be improved.
In the above-mentioned classifier, it is preferable that the conical member further comprise at least one ring-shaped member with a predetermined diameter and a predetermined thickness at a lower portion or the conical member. This is because by the provision of the ring-shaped member at the lower portion of the conical member, the flow of the powder material under the conical member can be changed in such a manner that the speed of the flow toward the center of the conical member is made greater than that in the other directions, whereby the introduction of the powder material to the central portion of the classification chamber can be facilitated and the deterioration of the classification performance of the classification chamber can be reduced.
When a plurality of the ring-shaped members is provided, the above-mentioned effect of reducing the deterioration of the classification performance of the classification chamber can be further increased.
In the above-mentioned classifier, it is preferable that at least one of the diameter or the thickness of the ring-shaped member be made changeable in accordance with the classification conditions. This is because when the diameter or the thickness of the ring shaped member is made changeable in accordance with the classification conditions, the yield of a desired product can be increased easily.
Furthermore, it is preferable that the ring-shaped member be made detachable from the conical member. This is because when the ring shaped member is made so as to be detachable from the conical member, the replacement of the ring-shaped member with a ring-shaped member with a different thickness or height can be carried out without difficulty in accordance with the desired classification performance, and the time required for the replacement can be shortened.
The second object of the present invention can be achieved by a method of producing toner for developing a latent electrostatic image to a visible toner image for use in electrophotographic image formation apparatus, wherein a toner with a predetermined particle diameter range is produced, including the step of classifying a pulverized solid material by use of the above-mentioned classifier of the present invention.